LOS ANGELES — A Batman bib-and-booties costume for $14.99 caught the eye of Joanna Robles as she searched for something for her months-old son to wear while she holds him as she hands out candy on Halloween.

“His dad is really obsessed with superheroes,” Robles said while strolling through a Spirit Halloween store outside Los Angeles.

In the next aisle, 35-year-old Joe Lige said he was browsing for an outfit “on the darker, spookier side” for a Halloween party. “My wife gives me a budget for Halloween and I always exceed it,” he quipped.

They’re among the 157 million Americans expected to celebrate Halloween this year, whether it’s by walking their kids around the block for free candy, carving a pumpkin for the front window or donning a costume for a neighborhood party.

Once mostly the purview of children, Halloween has grown into a major consumer holiday that now includes 18- to 34-year-old millennials and older adults who seize the opportunity for a night of escapism.

“It’s not just for kids trick-or-treating anymore,” said Trisha Lombardo, a spokeswoman for Spirit Halloween, which has 1,150 temporary stores that operate just for the Halloween season.

Consumers altogether will spend $6.9 billion on Halloween this year, or an average of $74.34 each, the National Retail Federation estimates based on an annual survey conducted by the research firm Prosper Insights & Analytics.

That’s down from a peak of $8 billion, or $79.82 a consumer, in 2012 but still more than double the Halloween spending of a decade ago.

That spending cuts a huge swath across the retail and entertainment economies, and to some extent the farming industry.

Businesses can’t point to a single reason why consumer interest in Halloween has surged over the last decade but they do cite factors driving its popularity today.

For instance, “the millennials are really into group costumes and activities,” Lombardo said. “They love to do things in groups, whether they’re going as characters in The Walking Dead or Orange Is the New Black,” and that drives added costume sales and theme-park attendance, she said.

Social media also has fueled the rise in Halloween’s appeal because consumers love sharing information, photos and videos of their Halloween costumes, decorations and night-on-the-town escapades, analysts said.

When people were asked where they look for inspiration for costumes, the websites Facebook and Pinterest each drew 13 percent of the responses, the National Retail Federation said. Nearly one-third of consumers said they looked online overall for Halloween costumes.

“I post stuff after I pick out my outfit,” especially on Facebook and Instagram, said Jessica Medina, 33, as she shopped at the Spirit Halloween store. “I also search online to see what’s out there and my friends post things.”

The impact of social media “has been exponential,” said Jeff Green, a Phoenix-based retail consultant. “The growth that’s occurred in Halloween spending over the last 10 years has almost mirrored that of the growth in social media. It’s more of a social holiday now, with ‘social’ meaning that people not only are enjoying Halloween with one another but sending it out to the world.”

Consumer spending on Halloween still pales next to the winter holiday season, which includes Thanksgiving and Christmas and generates about $616 billion in sales. Halloween spending also trails spending on Mother’s Day, Valentine’s Day and Father’s Day.

That’s mainly because “Halloween is not a gift-giving holiday” much beyond candy for the kids, said National Retail Federation spokeswoman Kathy Grannis Allen.

Nonetheless, “it’s still a huge business for a retailer,” she said, although Halloween retail sales are expected to dip this year compared with 2014 in part because many consumers are re-using Halloween props and decorations they’d bought in recent years.

“We really think it comes down to several years of stocking up on Halloween goods and this year using more of what they already have,” Allen said. Spending on decorations is expected to total $1.9 billion this year, down 7 percent from last year.

“We’re not disappointed with what we’re seeing with Halloween this year in any fashion,” she said.